In an electrophotographic image formation process, an image is formed by developing an electrostatic latent image formed on a photoreceptor with a toner containing a colorant, transferring the toner image onto transfer paper, and fixing the toner with a heat roll, etc. The photoreceptor is cleaned for the next cycle of electrostatic image formation. Dry developers used in such electrophotography are divided into a one-component developer that is a toner itself comprising a binder resin having a colorant dispersed therein and a two-component developer comprising such a toner and a carrier mixed therewith. Upon carrying out copying operations using these developers, excellent fluidity, transportability, fixability, chargeability, transfer properties, and cleanability are required for ensuring process suitability.
To meet the demand for compact equipment for space saving, an image formation system in which a residual toner is recovered simultaneously with development to omit a cleaning step has been proposed recently (hereinafter referred to as a "cleanerless system") (see JP-A-5-94113, the term "JP-A" as used herein means an "unexamined published Japanese patent application"). This system has the disadvantage that the recovered toner is different from the other part of the toner in charging characteristics so that it is not transferred and remain in the developing unit. Therefore, the cleanerless system has been demanded to have improved transfer efficiency.
Separately, it has been proposed to make toner particles almost spherical in order to improve the fluidity, chargeability, and transfer properties. However, use of spherical or nearly spherical toner particles causes the following problems. A developing unit is equipped with a transport control plate, and the amount of a developer to be transported can be controlled by adjusting the distance between the transport control plate and a magnetic roll. The problem is that the rate of the change in the transported developer amount to the change in the distance between the magnetic roll and the transport control plate for adjusting the transported developer amount increases as the shape of toner particles approach spheres. As a result, the transported amount becomes unstable. Such a problem can be suppressed by making all toner particles shapeless, but this causes reductions in fluidity and transfer efficiency and changes in chargeability and fluidity with time due to external additive's migrating and embedding into depressions of the toner particles.
Proposals have been made to obtain a developer satisfying all the requirements for fluidity, chargeability, transportability, and transfer properties by regulating the range of the shape of toner particles. For example, JP-A-61-279864 discloses a toner, shape of which is limited so that the median of the shape index may fall within a specific range. However, even with the regulated median of the shape index, sufficient transfer efficiency cannot be secured if shapeless toner particles exceed a certain proportion. Where the shapeless particles have small size, transfer becomes more difficult.
JP-A-1-185654 teaches that the rise in toner charging and charge distribution can be sharpened by regulating the relationship between the median of the shape of a toner and that of a carrier. Taking into account the distribution of the particle diameter and particle shape, however, sufficient transfer efficiency applicable to a cleanerless system cannot be obtained if the content of small and shapeless toner particles exceeds a certain proportion. In addition, insufficient cleaning and transport are caused.
Hence, it has been proposed to regulate the proportion of nearly spherical particles in number and the proportion of shapeless particles in number to improve cleanability, developing properties, and image quality (see JP-A-6-148926 and JP-A-6-148941). Further, JP-A-8-328312 proposes achieving both desired transfer properties and image quality by making black toner particles more shapeless than the other three color toners while making the other three color toners spherical. However, transfer properties change depending not only on shape but also on size. That is, small diameter toner particles and shapeless toner particles are hard to transfer due to strong electrostatic adhesion to a photoreceptor. Therefore, if the shapeless particles have a small diameter, sufficient transfer efficiency for application to a cleanerless system cannot be obtained even though the proportion of the number of the shapeless particles is reduced.
On the other hand, in order to cope with high-speed and large number of sheets copying systems while fulfilling the recent demand for color printing, especially on-demand color printing, a system comprising forming a multi-color image on a transfer belt and transferring and fixing the multi-color image onto an image fixing material at a time has been reported (see JP-A-8-115007).
Taking the step of transferring a toner image from a photoreceptor to a transfer belt as first transfer and the step of transferring the multi-color image from the transfer belt to an image fixing material as second transfer, there remains an untransferred toner in both the first and second transfer steps, which reduces the overall transfer efficiency and, of course, necessitates a cleaning step.
Particularly in the second transfer step, where a multi-color image is transferred all at once, and the image fixing material varies, for example in the case of paper, in terms of its thickness and surface properties, it has now been an outstanding subject to improve transfer properties and cleanability of an untransferred remaining toner. In this connection, the shape of a toner has been attempted to be controlled in order to improve its fluidity, chargeability, transportability, transfer properties, and cleanability. For example, to make toner particles spherical or nearly spherical has been proposed so as to improve fluidity, chargeability, and transfer properties, but cleanability is reduced as toner particles approach spheres. While a cleanerless system has been proposed as described above, in which a residual toner is recovered simultaneously with development while bringing the transfer efficiency as close to 100% as possible, this system is difficult to apply to full color printing because four color toners would be mixed with each other.
Further, with the spread of color printers based on electrostatic latent image development, it has been desired for the printers to be applicable to not only specific paper for exclusive use but a variety kinds of paper. When common paper is used, there are tendencies that paper dust remains on a photoreceptor to interfere with latent image formation or enters a developing unit to reduce the developing performance, leading to image missing.
Since sufficient cleanability cannot be secured simply by regulating the median of a shape index, regulating the proportion of nearly spherical particles in number has been proposed (see JP-A-6-148926 and JP-A-6-148941). However, transfer properties change not only with shape but also with size as previously stated. Therefore, a toner should be designed with due consideration for both of particle shape and size in order to obtain sufficient cleanability while securing satisfactory transfer properties.